Infants under three months haven't yet developed their immune systems to fight infections, so at a temperature of 100.4F or higher, you need to call the doctor. For three to six-month-olds, it's also a good idea to consult your doctor if concerned.

If an older child has a fever, consider more than the number on the thermometer. Pay attention to how your child looks and to his behavior. If she's lethargic and unresponsive or not eating, consider contacting your doctor. If, on the other hand, your child is running around, playing, eating and acting like herself, then a doctor's visit may not be necessary.

Other situations in which it's best to contact a doctor: if the fever persists more than three days, if the child hasn't been immunized (which increases the risk of bacterial infections), or if your child is immune compromised (HIV, cardiac trouble).